膠質細胞の電子顕微鏡的研究

Electron Microscopic Studies on the Neuroglia Cell

いとぐち

　神経膠質組織の構造に関しては，前世紀末より今世紀前半にわたる間に，可視光顕微鏡（以下「光顕」と略記）検索の為の，多数の優秀な組織学的手技が創案せられ，これによつて，特殊部位のものは除いて，一般には膠質細胞は3種の主型，すなわち星状膠細胞・稀突起膠細胞・小膠細胞に区分せられだ。この間世に問われた報告は極めて多く，Spielmeyer（1922）^1），Penfield（1932a^2），1932b^3）），Del Rio-Hortega（1932）^4），Glees（1955）^5）等によつて検討概括された。しかしながらGolgi氏法や各種の銀法では，膠質細胞は黒色に鍍金され，その内部構造を見ることが出来ず，色素染色による場合でも，従来の神経組織固定法の不備や，光顕分解能の限界に禍され，細胞内の超微細構造や，周囲の組織細胞との関係については，不確実な所見が多い。

　膠質細胞は中枢神経系内のneuronと血管との中間に位置し，為に所謂血脳関門の観点から，近時注目を沿びている（本陣，1960a^6）参照）。またneuronの新陳代謝や，神経の興奮伝導機序への関与についても検索を必要とする点が少くない。これらの問題の解明の為にも，神経膠組織の微細構造を明らかにすることが先ず必要と思われる。以下著者の得た超薄切片に基く電子顕微鏡（以下「電顕」と略記）所見について報告する。

　The fine structure of the neuroglia cells and the vascular bed of the central nervous system has been studied in the cerebrum, cerebellum and spinal cord of adult mice and frogs. The 1% OsO₄ and KMnO₄, small pieces of materials were fixed in 1% OsO₄, and 0.6% KMnO₄, solution amended with veronal-acetatt buffer (pH 7.25), embedded in plastic and sectioned with glass knives. Electron micrographs were taken an original magnification of 2,000 to 30,000 and thereafter enlarged photographically. The identification of neuroglia cell types has been based chicily on the classical classification in the light microscope literature summarized by Penfield (1932) and Del Rio-Hortegit (1932).

　In the central nervous system, the cell bodies and processes of the neurons and neuroglia cells are fitted closely together to fill substantially all of the available space.The capillaries are put between these neural components, The neurons, neuroglia and capillaries are contiguous with (ma another. An electron less-dense, thin layer of only 100 to 200 Å is found between adjacent elements. The so-called extracellular apace in light microscopy cannot be found; it Mil V be an artifact caused by poor fixation.

　The connective tissue derived from the leptomeningeum cannot he found in the parenchyma of the central nervous system, except around the large vessels. The capillaries are found to be closely sheathed by the processes of the neuroglia cells.

　The astrocytes are characterized in electron micrographs by their pale cytoplasm which contains in itself a small amount of endoplasmic reticulum and fine granules. A few number of mitochondria and Golgi membranes are also seen in the pericaryon. The processes of the astrocytes show a marked low electron density and contain in itself a few number of small granules and large mitochondria. They show to have a definite tendency to form sheathswhich cover most of the surfaces of the small blood vessels, as perivascular end-feet. Some of the processes of astrocytes reach the outer surfaces of the nerve cells, nerve fibers and the other kinds of neuroglia cells. The chromatin granules in the nucleus are few in number and show a clumped appearance. The nuclear membrane appears as a double membrane structure and has, at places, pore-like structure, nuclear pore, through which the small granules of the nucleolus are ranged from the nucleoplasm to the cytoplasm. Some of the astrocytes contain fine filamentous materials of about 100Å in diameter in their cytoplasm and procerses.

　The oligodendrocytes are identified by their moderately dense cytoplasm similar to that of the neurons except for the absence of neurofilaments. The cytoplasm contains moderate quantities of endoplasmic reticulum, a considerable number and density of fine granules, a moderate number of mitochondria and a few sparse groups of the Golgi membranes. The processes of oligodendrocytes spread in thin sheets between the bundles of nerve fibers. The limiting membrane of the processes is in continuous connection with the myelin lamellar membranes of the nerve fibers. Some of the processes reach to the Outer surfaces of the nerve cell bodies and capillaries. The chromatin granules are in moderate numbers. The nuclear membrane appears as a double membrane and has many nuclear pores.

　The microglial cells are distinguished in electron micrographs because of their extreme overall density and angular, indented contour. The chromatin granules are much more numerous. The nucleoli, consisting of many fine granules, are conspicuous. The cytoplasm of microglial cells is stuffed with many dense granular materials. A large amount of endoplasmic reticula and clumps of the Golgi membranes and a moderate number of mitochondria can be found in the cytoplasm Theprocesses of microglial cells are also extremely dense and show an angular contour, spreading between other cell processes.

　In the central nervous system, many myelinated nerve fibers exist in close apposition, coming in contact with each other at the outer surface of their myelin sheaths. The myelin membrane is connected with the limiting membrane of the process of oligodendrocyte, after winding itself helically about the axon. It has been often found that the outer end of helical myelin membrane of a myelinated fiber is in direct continuous connection with that of an adjacent one. Besides the oligodend rocytes and their processes, there are found large and small processes of both the astrocytes and microglial cells between the nerve fibers.

　At the inside and outer surface of all sorts of the neuroglia cells and their processes, there is seen no fibrilar structure, which corresponds to the neuroglia fiber described by light microscopy, in so far as normal materials we used.

　The capillary endothelium forms a continuous layer, though considerably attenuated. The cytoplasm of the endothelial cells contains moderate quantities of endoplasmic reticula, granules and mitochondria. Fenestration of the endothelium of capillaries observed in some other parts of the body cannot he found. Theendothelium is surrounded by a thin basement membrane. Along the wall of capillaries, there are sporadically found pericytes which are inserted between the endothelium and the basement membrane. The basement membrane exists also between the endothelium and pericytes. Therefore, the pericyte is surrounded by the basement membranes at its inner and outer surfaces.

　The astrocytic perivascular end-feet, having a characteristically low density, surround the most part of the capillary wall. They form a single layer around the capillaries, though having no constant thickness, and do not overlap one another to any extent. It appears that the capillaries are almost engulfed in less electron dense processes of the astrocytes. This perivascular sheath of the processes of astrocytes is not a complete one, but several other glial elements, such as processes of oligodendrocytes and microglial cells, come in contact with a part of the outer surface of capillaries. The perivascular, neuroglial sheath is in contact with the basement membrane of the capillary ; there is found neither pial connective tissue' nor perivascular space described in light microscopy, between the capillary and the neuroglia processes.